How to avoid “header already sent” warning when loading a pdf

Question

In drupal 7, when I load a pdf page in a new tab, and after reloading the initial page, I have this warning:

Warning : Cannot modify header information - headers already sent in invoices_callback() (line 1093 in C:\xampp\htdocs\drupal\sites\all\modules\custom_profile\custom_profile.module).
Warning : Cannot modify header information - headers already sent in drupal_send_headers() (line 1221 in C:\xampp\htdocs\drupal\includes\bootstrap.inc).

Actually, I have a link in my initial page (named 'my-account/invoices') which redirects to a PDF page named 'my-account/invoices/%invoice_number'. This last one is loaded by a 'page callback' function :

function invoices_callback($var){
require_once('sites/all/libraries/dompdf/dompdf_config.inc.php');

$filename = 'Invoice n°' . $var . '.pdf';
$dompdf = new DOMPDF();
    $dompdf->load_html('<div>Test n° : ' . $var . '</div>');
    $dompdf->render();
    $dompdf->stream($filename, array('Attachment'=>0));
    header('Content-type: application/pdf');

}

As you see, I didn't use a pdf module, like printer, which is too heavy. I use a custom module which implement a page callback function, and inside I use dompdf library.
The pdf is displayed normally, but when I come back to initial page I have these warnings.

I have tried several things:

• I removed the header function. Effect: the first warning is gone but I still have the second. It seems like the stream method of DOMPDF class wants to sent a header...
• I have added a charset utf-8 to the header function
• I used ob_start() and ob_end_flush() functions at the beginning and at the end of the code. Surprisingly doesn't work.
• I lastly use drupal_add_http_header function. Doesn't remove warnings.

What i can do is to call a script in an external link, because my test work when i call a php script located elsewhere. But for the UX, i try to have all my scripts managed by drupal, and i want the invoices to be called by a simple alias url (my-account/invoices/%invoice_number).

So my question is simple: How to avoid these warnings without moving my script to an external location?

Answer 1

So, this question is very specific. The problem of 'headers already sent' warning is common, and each situation is specific, but here...

I don't need to send header-content-pdf, it is redundant, because the stream method of dompdf object sent it. So I had to erase this line.

Secondly, the most important thing is that: My code is in a page callback function, which is executed by drupal at a moment of the page generation process. After a page callback function is executed, the http headers are not sent yet, it is the job of the delivery callback function which is by default drupal_deliver_html_page. We can override this default behavior by adding in the hook_menu item the delivery callback property:

"delivery callback": The function to call to package the result of the page callback function and send it to the browser. Defaults to drupal_deliver_html_page() unless a value is inherited from a parent menu item. Note that this function is called even if the access checks fail, so any custom delivery callback function should take that into account. See drupal_deliver_html_page() for an example.

Implementing our custom delivery callback function should work... But I didn't try it... Because, in my research, i found this function: drupal_exit()

Performs end-of-request tasks. In some cases page requests need to end without calling drupal_page_footer(). In these cases, call drupal_exit() instead. There should rarely be a reason to call exit instead of drupal_exit();

But here, for me there is a reason to end the request roughly: The stream method of the dompdf object render the page as i want, ie a pdf page. So to be clear, here is my new code:

function invoices_callback($var){
require_once('sites/all/libraries/dompdf/dompdf_config.inc.php');

$filename = 'Invoice n°' . $var . '.pdf';
$dompdf = new DOMPDF();
$dompdf->load_html('<div>Test n° : ' . $var . '</div>');
$dompdf->render();
$dompdf->stream($filename, array('Attachment'=>0));
drupal_exit();

}

The stream method sends the http header and renders the page, we don't need anything else, so we exit. It takes me a lot of time, i know how drupal generates a page now. Hope this helps.

Answer 2

You can disable all messages , execute the instructions prone to failure and reactivate the messages to restore stability to the system.

    // Desactivamos los posibles warning para que no se envien los headers
    error_reporting(0);

$dompdf = new DOMPDF();
$dompdf->load_html($html);  
$dompdf->render();

    // Save file    
    $output = $dompdf->output();
    $dest = "/your-directory/final_file.pdf";        
    $result = file_put_contents($dest, $output);        

    // Reactivamos los warning            
    error_reporting(E_ALL ^ E_NOTICE);                        

    if( $result ) {
        // Send file       
        $dompdf->stream($dest);            
    }
    else{            
         echo "<p> An unexpected error has occurred. Please try again later or contact support. </p>";          
    }